It is known to provide an exhaust system with an exhaust gas recirculation device (EGR device). In these devices, exhaust gas of a vehicle flowing in the exhaust system of a vehicle is mixed with intake air flowing in the intake pipe to thereby reduce the gas temperature. As such, harmful material (e.g., nitrogen oxide) in the exhaust gas is reduced. However, such recirculation (i.e., return) of the exhaust gas can cause reduction of the output and drivability of the internal combustion engine. Thus, it may be necessary to adjust the volume of exhaust gas recirculated from the exhaust pipe to the intake pipe.
In partial response to this need, exhaust systems have been proposed that include an exhaust gas recirculation volume control valve (EGR control valve) that controllably changes the open area of an exhaust gas recirculation passage to thereby adjust the volume of recirculated exhaust gas. One example of this conventional EGR control valve includes a butterfly type valve mounted on the tip of the valve shaft and transmits the rotational motion of an actuator (e.g., an electric motor with a gear speed reduction mechanism) to rotate the butterfly type valve around the valve shaft between an open and a closed position. More specifically, a circular ring-shaped groove is formed in the outer peripheral surface of the butterfly type valve, and a seal ring is fitted in this seal ring groove to prevent a leak of high-temperature fluid when the valve is closed. In other words, when the valve is closed, the seal ring expands radially to thereby seal against the inner peripheral surface (i.e., the seal surface) of the circular pipe-shaped liner, which houses the butterfly type valve.
The seal ring may be warped due to differences in thermal expansion between the seal ring and the liner. In order to reduce the likelihood of warping, a radial cutout clearance can be provided in the seal ring. However, multiple seal rings are provided that overlap each other, and the seal rings are rotated relative to each other such that the cutout clearance of each is misaligned. As such, fluid leakage through the cutout clearances is unlikely. Also, a backup ring is provided between the two seal rings so as to prevent fluid leakage through the bottom of the seal ring groove and through the cutout clearances. (See for example, Japanese Patent Publication No. 60-245,880.)
However, this type of EGR control valve can be expensive due to the multiple, expensive seal rings and the backup ring included therein. Also, maintaining the seal rings rotated relative to each other may require expensive components. Moreover, in order to fit the backup ring in the seal ring groove, the bottom surfaces of two butterfly plates are assembled into close contact with each other to construct a circular disk-shaped butterfly type valve. That is, the butterfly type valve is constructed of two parts of two butterfly plates. This makes the structure complex and increases the number of parts. Thus, assembly can be complicated and costs can be excessive.